I don't think I've ever heard of the term "overview" used in this context.
I have heard of agents wanting both a summary and a synopsis and I suspect that's what the agent wants. A summary (or overview) is just a stripped-to-the-bare-bones synopsis of what the book is about. If you've never written one, try this exercise: assume someone in a grocery store checkout lane has asked you what your book is about. You've only got half a minute to do it. What do you tell her?